The formation of stones in the kidneys or urinary tract is not an uncommon disorder. The stones are formed from the chemicals usually found in the urine such as uric acid, phosphorus, calcium, and oxalic acid. They may vary in consistency from grit, sand, and gravel-like obstructions the size of a bird's egg. Stones may form and grow because the concentration of a particular substance in the urine exceeds its solubility. Most kidney stones are composed either of calcium oxalate or phosphate, the latter being most common in the presence of infection. About ninety per cent of all stones contain calcium as the chief constituent. More than half of these are mixtures of calcium, ammonium and magnesium, phosphates and carbonates, while the remainder contain oxalate.